A magnetic resonance imaging (MRI) device is a medical-use image capturing system that generates nuclear magnetic resonance in hydrogen nuclei within any plane traversing a test subject, and takes a tomographic image within the plane, based on nuclear magnetic resonance signals being generated. In general, a slice gradient magnetic field is applied for identifying an imaging plane, simultaneously with providing exciting pluses that excite magnetization within the plane. Accordingly, nuclear magnetic resonance signals (echoes) are obtained, which are generated at a stage of convergence of magnetization that has been excited. In addition, a phase encoding gradient magnetic field and a frequency encoding gradient magnetic field, being orthogonal to each other within the tomographic plane, are applied for providing the magnetization with positional information, during a period from the excitation until obtaining the echoes.
The pulses and each of the gradient magnetic fields for generating echoes are applied according to a predetermined pulse sequence. Various pulse sequences are known depending on purposes. For example, there is a gradient echo (GE) type high-speed imaging method that applies gradient magnetic fields while applying exciting pulses repeatedly at relatively short intervals, and acquires echoes.
In this high-speed imaging method, a trapezoid wave of the gradient magnetic field, having nearly maximum amplitude, is turned on and off at high speed, and therefore, extremely loud noise, 100 dB or more, is generated within a bore where the test subject is placed. This noise has loudness considerably jarring the test subject placed in the bore, even though the test subject wears headphones or earplugs. Since this type of noise becomes louder, as a level of magnetization becomes higher, countermeasures are needed against a high magnetic-field machine of 3 T (tesla) or higher.
As one of the countermeasures, there is a method of acoustic noise reduction by allowing the trapezoidal wave of the gradient magnetic field to pass through a low-pass filter, and smoothing variation of amplitude at a rise time and a fall time of the wave (e.g., see Non Patent Document 1). In addition, sound generated by the gradient magnetic field is expressed by a product of a frequency distribution of the gradient magnetic field waveform and a frequency response function (FRF) inherent to the device. By utilizing this feature, there is a method of using a low-pass filter to reduce a frequency component of the gradient magnetic field in a range where the FRF exceeds a minimal level, thereby canceling the noise (e.g., see Non Patent Document 2).